Some scientists have proposed that previously unknown natural causes could be responsible for global warming, such as a decrease in cloud-producing galactic cosmic rays. Others have proposed that the climate may be responding to the heat-trapping effects of carbon dioxide by producing more clouds, which reflect away sunlight and offset the added heat-trapping gases. These theories have little support among actively publishing climate scientists. Despite public belief that climate scientists are divided about the human contribution to our changing climate, polling data show high agreement among climate scientists that humans are significantly affecting the climate. A 2008 poll of actively publishing climate scientists found that 97% said yes to the question, "Do you think human activity is a significant contributing factor in changing mean global temperatures?" In my personal experience interacting with climate scientists, I have found near-universal support for this position. For example, I am confident that all 23 climate scientists and meteorologists whom I am personally acquainted with at the University of Michigan's Department of Atmospheric, Oceanic, and Space Science would agree that "human activity is a significant contributing factor in changing mean global temperatures." It is good that we have scientists skeptical of the prevailing consensus challenging it, though, because that is how scientific progress is made. It may be that one of the scientists making these challenges will turn out to be the next Einstein or Galileo, and overthrow the conventional scientific wisdom on climate change. But Einsteins and Galileos don't come along very often. The history of science is littered with tens of thousands of discredited scientific papers that challenged the accepted scientific consensus and lost. If we rely on hopes that the next Einstein or Galileo will successfully overthrow the current scientific consensus on climate change, we are making a high-stakes, low-probability-of-success gamble on the future of civilization. The richest and most powerful corporations in world history, the oil companies, have spent hundreds of millions of dollars to push us to take this gamble, and their efforts have been very successful. Advertising works, particularly when your competition has little money to spend to oppose you.

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Our recent unusual weather has made me think about this a lot. The natural weather rhythms I've grown to used to during my 30 years as a meteorologist have become significantly disrupted over the past few years. Many of Earth's major atmospheric circulation patterns have seen significant shifts and unprecedented behavior; new patterns that were unknown have emerged, and extreme weather events were incredibly intense and numerous during 2010 - 2011. It boggles my mind that in 2011, the U.S. saw 14 - 17 billion-dollar weather disasters, three of which matched or exceeded some of the most iconic and destructive weather events in U.S. history--the "Super" tornado outbreak of 1974, the Dust Bowl summer of 1936, and the great Mississippi River flood of 1927. I appeared on PBS News Hour on December 28 (video here) to argue that watching the weather over the past two years has been like watching a famous baseball hitter on steroids--an analogy used in the past by climate scientists Tony Broccoli and Jerry Meehl. We're used to seeing the slugger hit the ball out of the park, but not with the frequency he's hitting them now that he's on steroids. Moreover, some of the home runs now land way back in the seats where no one has ever been able to hit a home run before. We can't say that any particular home run would not have occurred without the steroids, but the increase in home runs and the unprecedented ultra-long balls are highly suspicious. Similarly, Earth's 0.6°C (1°F) warming and 4% increase in global water vapor since 1970 have created an atmosphere on steroids. A warmer atmosphere has more energy to power stronger storms, hotter heat waves, more intense droughts, and heavier flooding rains. Natural weather patterns could have caused some of the extreme events we witnessed during 2010 - 2011, and these years likely would have been naturally extreme years even without climate change. But it strains the bounds of credulity that all of the extreme weather events--some of them 1-in-1000-year type events--could have occurred without a signicant change to the base climate state. Mother Nature is now able to hit the ball out of the park more often, and with much more power, thanks to the extra energy global warming has put into the atmosphere.

Extreme weather years like 2010 and 2011 are very likely to increase in frequency, since there is a delay of several decades between when we put heat-trapping gases into the atmosphere and when the climate fully responds. This is because Earth's oceans take so long to heat up when extra heat is added to the atmosphere (think about how long it takes it takes for a lake to heat up during summer.) Due to this lag, we are just now experiencing the full effect of CO2 emitted by the late 1980s; since CO2 has been increasing by 1 - 3% per year since then, there is a lot more climate change "in the pipeline" we cannot avoid. We've set in motion a dangerous boulder of climate change that is rolling downhill, and it is too late to avoid major damage when it hits full-force several decades from now. However, we can reduce the ultimate severity of the damage with strong and rapid action. A boulder rolling downhill can be deflected in its path more readily early in its course, before it gains too much momentum in its downward rush. For example, the International Energy Agency estimates that every dollar we invest in alternative energy before 2020 will save $4.30 later. There are many talented and dedicated people working very hard to deflect the downhill-rolling boulder of climate change--but they need a lot more help very soon.

tens of millions of people stand to see their lives ruined because the bureaucrats at the ECB don't understand introductory economics -- Dean Baker